<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    // 数组每项相加。
    let res = [[1,2], [1,2,3,4]].map((item) => {
      let num = 0
      if(item.length === 1) {
        return item[0]
      } else {
        for(let i = 0; i < item.length; i++) {
          num += item[i]
        }
      }
      return num
    })
    console.log(res)
  </script>
</body>
</html>